Compared to inorganic compounds, organic compounds include more varied material systems, and through appropriate molecular design it is possible to synthesize organic materials having various functionalities. Further, the organic compound is characterized in that films and the like formed using the organic compound demonstrate great pliancy, and superior processability can also be achieved by polymerization. In light of these advantages, in recent years, attention has been given to photonics and electronics employing functional organic materials.
Photonic techniques which make use of photophysical qualities of organic compounds have already played an important role in contemporary industrial techniques. For example, photosensitive materials, such as a photoresist, have become indispensable in a photolithography technology used for fine processing of semiconductors. In addition, since the organic compounds themselves have properties of light absorption and concomitant light emission (fluorescence or phosphorescence), they have considerable applicability as light emitting materials such as laser pigments and the like.
On the other hand, since organic compounds do not have carriers themselves, they essentially have superior insulation properties. Therefore, in the field of electronics where the electrical properties of organic materials are utilized, the main conventional use of organic compounds is insulators, where organic compounds are used as insulating materials, protective materials and covering materials.
However, there are means for making massive amounts of electrical current flow in the organic materials which is essentially insulators, and they are starting to be put to practical use in the electronics field. The means discussed here can be broadly divided into two categories.
The first of these, represented by conductive polymers, is means in which a π-conjugate system organic compound is doped with an acceptor (electron acceptor) or a donor (electron donor) to give the π-conjugate system organic compound a carrier “Synthesis of Electrically Conducting Organic Polymers: Halogen Derivatives of Polyacetyrene, (CH)x)” by Hideki Shirakawa et al., Chemical Communications, 1977, 16, 578-580. By increasing the doping amount, the carrier will increase up to a certain area. Therefore, its dark conductivity will also increase together with this, so that significant electricity will be made to flow.
Since the amount of the electrical flow can reach the level of a normal semiconductor or more, a group of materials which exhibit this behavior can be referred to as organic semiconductors (or, in some cases, organic conductors).
This means of doping the acceptor/donor to improve the dark conductivity to make the electrical current flow in the organic material is already being applied in part of the electronics field. Examples thereof include a rechargeable storage battery using polyaniline or polyacene and an electric field condenser using polypyrrole.
The other means for making massive electrical current flow in the organic material uses an SCLC (Space Charge Limited Current). The SCLC is an electrical current which is made to flow by injecting a space charge from the outside and moving it, the current density of which is expressed by Child's Law, i.e., Formula (1), shown below. In the formula, J denotes a current density, ∈ denotes a relative dielectric constant, ∈0 denotes a vacuum dielectric constant, μ denotes a carrier mobility, V denotes a voltage, and d denotes a distance (herein after, referred to as “thickness”) between electrodes applied with the voltage V:[Formula 1]J=9/8·∈∈0μ·V2/d3  (1)
Note that the SCLC is expressed by Formula (1) in which no carrier trap when the SCLC flows is assumed at all. The electric current limited by the carrier trap is referred to as a TCLC (Trap Charge Limited Current), and it is proportionate to a power of the voltage, but both the SCLC and the TCLC are currents that are subject to bulk limitations. Therefore, both the SCLC and the TCLC are dealt with in the same way hereinbelow.
Here, for comparison, Formula (2) is shown as a formula expressing the current density when an Ohm current flows according to Ohm's Law. σ denotes a conductivity, and E denotes an electric field strength:[Formula 2]J=σE=σ·V/d  (2)
In Formula (2), since the conductivity C is expressed as σ=neμ (where n denotes a carrier density, and e denotes an electric charge), the carrier density is included in the factors governing the amount of the electrical current that flows. Therefore, in an organic material having a certain degree of carrier mobility, as long as the material's carrier density is not increased by doping as described above, the Ohm current will not flow in a material which normally does not have few carriers.
However, as is seen in Formula (1), the factors which determine the SCLC are the dielectric constant, the carrier mobility, the voltage, and the thickness. The carrier density is irrelevant. In other words, even in the case of an organic material insulator with no carrier, by making the thickness d sufficiently small, and by selecting a material with a significant carrier mobility μ, it becomes possible to inject a carrier from the outside to make the current flow.
Even when this means is used, the current flow amount can reach the level of a normal semiconductor or more. Thus, an organic material with a great carrier mobility μ, in other words, an organic material capable of latently transporting a carrier, can be called an organic semiconductor.
Incidentally, even among organic semiconductor elements which use the SCLC as described above, organic electroluminescent elements (herein after, referred to as “organic EL elements”) which use both the photonic and electrical qualities of functional organic material as photoelectronic devices, have particularly demonstrated remarkable advancement in recent years.
The most basic structure of the organic EL element was in year of 1987. “Organic Electroluminescent Diodes” by C. W. Tan et al., Applied Physics Letters, Vol. 51, No. 12, 913-915 (1987). The element reported in Non-patent document 2 is a type of diode element in which electrodes sandwich an organic thin film having a total thickness of approximately 100 nm and being constituted by laminating a hole-transporting organic compound and an electron-transporting organic compound, and the element uses a light emitting material (fluorescent material) as the electron-transporting compound. By applying voltage to the element, light-emission can be achieved like a light emitting diode.
The light-emission mechanism is considered to work as follows. That is, by applying the voltage to the organic thin film sandwiched by the electrodes, the hole and the electron injected from the electrodes are recombined inside the organic thin film to form an excited molecule (herein after, referred to as a “molecular exciton”), and light is emitted when this molecular exciton returns to its base state.
Note that, types of molecular excitons formed by the organic compound can include a singlet excited state and a triplet excited state, and the base state is normally the singlet state. Therefore, emitted light from the singlet excited state is referred to as fluorescent light, and the emitted light from the triplet excited state is referred to as phosphorescent light. The discussion in this specification covers cases of contribution to the emitted light from both of the excited states.
In the case of the organic EL element described above, the organic thin film is normally formed as a thin film having a thickness of about 100 to 200 nm. Further, since the organic EL element is a self-luminous element in which light is emitted from the organic thin film itself, there is no need for such a back light as used in a conventional liquid crystal display. Therefore, the organic EL element has a great advantage in that it can be manufactured to be extremely thin and lightweight.
Further, in the thin film having a thickness of about 100 to 200 nm, for example, the time from when the carrier is injected to when the recombination occurs is approximately several tens of nanoseconds, given the carrier mobility exhibited by the organic thin film. Even when the time required by for the process form the recombination of the carrier to the emission of the light, it is less than an order of microseconds before the light emission. Therefore, one characteristic of the organic thin film is that response time thereof is extremely fast.
Because of the above-mentioned properties of thinness and lightweightness, the quick response time, and the like, the organic EL element is receiving attention as a next generation flat panel display element. Further, since it is self-luminous and its visible range is broad, its visibility is relatively good and it is considered effective as an element used in display screens of portable devices.
The organic EL element has excellent features described above, on the other hand, the reason why it is not yet widely put into actual use is a drawback that the life of the element is not sufficiently long.
In an electroluminescent film constructing the organic EL element, the deterioration of function of its organic semiconductor is accelerated by the passage of electric current. It is known that in the organic EL element, the life of the element (half-life of luminescent luminance) deteriorates in a manner nearly inversely proportional to initial luminance, in other words, inversely proportional to the amount of electric current to be passed. The Japan Society of Applied Physics, Journal of Molecular Electronics and Bioelectronics, Vol. 11, No. 1 (2000), 86-99.
From this fact, it can be said that to decrease the amount of electric current passing through the electroluminescent film of the organic EL element is important not only from the viewpoint of power consumption but also from the viewpoint of the life of the element.
Therefore, an object of the invention is to provide an element structure that reduces the amount of electric current passing through the electroluminescent film of an organic EL element to improve the life of the element.